Test against valid and invalid US TIN prefixes

[dspinellis]

The existing isTaxId validation tests only checked the format,
while the validation code also tests for valid (US campus)
prefixes. The added tests, check that the prefix validations
actually work.

This has been done as a safety measure for the code's refactoring and the support of another locale. I will issue these in further PRs.

Checklist

• PR contains only changes related; no stray files, etc.
• README updated (where applicable)
• Tests written (where applicable)

[codecov]

Codecov Report

Merging #1408 into master will not change coverage.
The diff coverage is n/a.

@@ Coverage Diff @@## master #1408 +/- ##
=========================================
Coverage 100.00% 100.00% =========================================
Files 95 95 Lines 1248 1248 =========================================
Hits 1248 1248 

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[profnandaa]

Thanks for your contrib! 🎉